Update labs/promql/solution.md

[alexx384]

Thank you for this amazing tutorial.
I'm a bit confused with the solution q5 - average failure rate by version for the problem Build a query to show the average failure ratio over the last five minutes, split by application version.

I’ve proposed a small fix or adjustment that might help. Perhaps you could take a look and suggest if I’m misunderstanding something?

Let me rewrite the previous query in a different way:

avg without(job, status, instance)(
    rate(
        fulfilment_requests_total{status="failed"}[5m]
    ) * on(instance) group_left(app_version) app_info
) / avg without(job, status, instance)(
    rate(
        fulfilment_requests_total{status="processed"}[5m]
    ) * on(instance) group_left(app_version) app_info
)

There we can substitute

rate(
        fulfilment_requests_total{status="failed"}[5m]
    ) * on(instance) group_left(app_version) app_info

with e. As we have just 3 microservices. We end up having <e1, e2, e3> instant vector.

Let's substitute

rate(
        fulfilment_requests_total{status="processed"}[5m]
    ) * on(instance) group_left(app_version) app_info
)

with p. As we have just 3 microservices. We end up having <p1, p2, p3> instant vector.

As the result we've got:

avg without(job, status, instance)(
  <
  {app_version="1.3.1", instance="fulfilment-processor-1:9110", job="fulfilment-processor", status="failed"} e1,
  {app_version="1.3.1", instance="fulfilment-processor-2:9110", job="fulfilment-processor", status="failed"} e2,
  {app_version="1.5.2", instance="fulfilment-processor-3:9110", job="fulfilment-processor", status="failed"} e3
  >
) / avg without(job, status, instance)(
  <
  {app_version="1.3.1", instance="fulfilment-processor-1:9110", job="fulfilment-processor", status="processed"} p1,
  {app_version="1.3.1", instance="fulfilment-processor-2:9110", job="fulfilment-processor", status="processed"} p2,
  {app_version="1.5.2", instance="fulfilment-processor-3:9110", job="fulfilment-processor", status="processed"} p3
  >
)

which can be rewritten as:

<avg(e1, e2), avg(e3)> / <avg(p1, p2), avg(p3)>

In mind my it feels it should be instead:

<avg(e1 / p1, e2 / p2>, avg(e3 / p3)>

So here is my proposal that is built on top of q4.